In radio frequency receivers, Gilbert-type mixers are preferably used for converting the radio frequency transmission signals to the baseband. Said mixers may be constructed using analog circuit technology, using bipolar technology or using metal oxide semiconductor, MOS, technology. At the output of such mixers or frequency converters, the down-converted output signal, which also still contains radio frequency residues in addition to the useful signal, is initially ready as an output current.
In order to provide the downstream signal processing chain with an output signal from which the radio frequency residues have been eliminated, it is customary additionally to convert the output current into a voltage at a resistor in the output current branch of the Gilbert mixer, so that the radio frequency residues can be effectively dissipated via capacitances. In the case of the small operating voltages customary in modern circuit integration technologies in CMOS, the resistor described greatly reduces the available voltage swing at the mixer transistors and thus the achievable power, conversion and signal-to-noise ratio of the mixer as a result of the voltage drop across such a resistor. However, without conversion of the mixer current into a voltage, the radio frequency residues would be forwarded to the downstream signal processing chain.
A mixer circuit of this type with a low-noise amplifier connected upstream is specified in FIG. 2 of the document A 2-V CMOS Cellular Transceiver Front-End, Steyaert et al., IEEE Journal of Solid-State Circuits, Vol. 35, No. 12, December 2000, pp. 1895 to 1907.
At resistors connected to the output of the mixer stage specified in FIG. 2, a differential voltage would be forwarded to downstream stages with a high input resistance, with the disadvantages described above, which voltage builds up at the nonreactive load resistors, which have a low resistance relative to the output resistors of the mixer M5, M6, and the parallel-connected load capacitances for generating a common-mode level for DC voltage offset compensation.
In order, for the reasons described above, to send the differential current of the mixer transistors to a downstream circuit with a low input resistance, this current has to be conducted via feedback resistors which are needed for the generation of the low input resistance (“virtual ground”). In order, however, at the same time to achieve the required RC time constants of 10 μs, for example, as are usually required for channel selection in the baseband, it would be necessary to construct disproportionately large capacitances, the integration of which cannot be realized in practice using CMOS technology.
In the case of the above-described direct conversion of the radio frequency transmission signals into the baseband, comparatively large currents, typically of the order of magnitude of a few milliamperes, are required in order to suppress the comparatively large 1/f noise in the case of MOS transistors. In the case of CMOS maximum voltages of 1 V, for example, this leads to load resistances of a few 100 Ω. An RC time constant of 10 μs would accordingly require capacitances of a few 10 nF, that is to say a chip area of approximately a few 10 mm2 which would be a cost-intensive solution in particular in the case of large-scale integrated CMOS circuits. Moreover, radio frequency components still present would additionally have to be eliminated from the output signal, which would likewise necessitate the provision of large capacitances, which would again require additional chip area.
The document DE 69 31 61 55 specifies a circuit arrangement with two current paths each comprising a cascode stage. The output current is divided between the two paths. Detection of the common-mode level with corresponding feedback is additionally provided.
It is an object of the present invention to specify an interface circuit for connection to an output of a frequency converter which forwards the baseband useful signals as current to downstream circuits but not the radio frequency residues, in order that the latter can be effectively dissipated via capacitances, compensates for the DC offset of the frequency converter, encompasses a large dynamic range, offers downstream circuits a defined output signal level, enables programmable gain without additional current, and at the same time can be produced cost-effectively.